The invention relates to a method of coating carbon composite materials to provide oxidation resistance in such materials.
Carbon composites consist of fibrous carbon material such as carbon or graphite fibers, woven into a porous skeleton and a non-fibrous carbon matrix which fills the pores in the woven skeleton. Carbon/carbon components have great potential for use in high temperature applications because they are strong materials, are of low density and exhibit strong fracture toughness values. Another unusual characteristic of a carbon/carbon composite is that its strength increases with increasing temperture to 5000.degree. F.
The major drawback for the wide spread use of carbon/carbon composites is their poor oxidation resistance at elevated temperatures. At temperatures above 800.degree. F. in the presence of oxygen, oxidation of the carbon occurs which results in a significant decrease in the mechanical properties of the carbon/carbon composite. Studies have shown that the tensile strength of the carbon/carbon composite may decrease up to 50 percent because of oxidation with correponding weight losses of only 10-15 percent.
Silicon carbide (SiC) coatings in many forms have been suggested, developed and used as an oxygen diffusion barrier to prevent oxidation of the carbon/carbon composite. The SiC coatings work extremely well for applications above 1600.degree. F. where the carbon/carbon does not undergo thermal cycling (i.e.) 150 hours a 1700.degree. F. The SiC provides oxidation protection for the carbon/carbon at these temperatures because the SiC will oxidize to SiO.sub.2 and the SiO.sub.2 will seal the cracks which develop in the SiC coating. Cracks develop in the SiC coating because of the thermal expansion difference between the SiC coating and the carbon/carbon composite. Below 1600.degree. F. the SiC does not provide oxidation protection because at these temperatures oxidation of the SiC to SiO.sub.2 will not occur. As a result, the cracks in the SiC coating are not sealed and the carbon/carbon composite is exposed to the oxidizing atmosphere.
Under conditions of thermal cycling the SiC cannot provide oxidation protection for carbon/carbon because each time the composite is heated and cooled the differences in thermal expansion coefficients between the carbon/carbon and SiC coating causes stresses to develop in the SiC coating. These streses increase both the number of cracks and/or the width of the existing cracks.
In severe thermal cycle spalling of the SiC coating can occur. The effect of the increased cracking and spalling is to expose portions of the carbon/carbon composite to the oxidizing atmosphere
A dual coating system was developed in an attempt to overcome the problems associated with the SiC coating. This dual coating system can consist of a conversion layer as well as a sealant material. The purposee of the conversion layer is to provide a gradient in the thermal expansion from the carbon body to the outer SiC layer. While the conversion layer improves the bonding of the SiC coating to the carbon/carbon/substrate and reduces some of the stresses in the coatings caused by thermal expansion mismatch, it does not prevent the SiC from cracking during thermal cycling.
In order to prevent oxidation through the cracks in the SiC coating the dual coatings provide a sealant material to flow into and seal the cracks which develop in the SiC outer coating. The sealant usually consists of a low temperture glass former.
However, these coating systems only provide oxidation protection to a carbon/carbon substrate for up to 350-450 hours under thermal cycling between 2500.degree. to 1200.degree. F.
The probable reason for failure is this dual coating system after only 350 hours of cycling is either the width of the cracks which develop in the SiC becomes too large for low temperature glassy phase to seal effectively and/or the quantity of glass phase sealant is not great enough to completely seal all of the cracks enabling large portion of the carbon/carbon composite to become exposed to the oxidizing atmosphere.
As mentioned previously, surface coatings methods utilized to protect the carbon/carbon composite from oxidation have not been successful because of cracking and spalling of the SiC coating- because of the large thermal gradients.
Conversion layers and glassy phase sealant layers have been only partially successful because once the coating is broken or the amount of glassy phase sealant is consumed, the underlying carbon composite material is vulnerable to oxidation. U.S. Pat. No. 4,582,751 teaches a method of providing additional oxidation protection in the form of an oxidation inhibitor within the pores of the carbon/carbon composite. Other methods for providing oxidation protection within the carbon/carbon composite. Other methods for providing oxidation protection within the carbon/carbon substrate are disclosed in Shaffer U.S. Pat. No. 4,321,298.
Thus, the current state of the art for oxidation protection systems consist of the following:
(a) inhibited carbon/carbon substrate PA1 (b) boron rich interlayers in dual layer systems. PA1 (c) SiC outer coatings.
Though this discussion will be limited to the use of boron with silicon carbide, as one progresses it will become clear that the inventive concept deals primarily with a material structure which may be applicable with other combinations of compatible materials.